Although the following description relates specifically to a welding apparatus, it must be understood that, depending on the laser beam parameters, the apparatus according to the present invention can also been used for machining operations, such as cutting, drilling or ablation of materials.
Systems for circular welding with a laser beam, for example, are known from the prior art.
Some of these systems are based on the principle that the welding apparatus is arranged with respect to one or more workpieces to be welded in a stationary manner. The laser beam is directed to a welding region of the workpieces in a fixed direction, whereas the workpieces are driven in rotation by a support comprising motor means for that purpose.
However, such systems present many drawbacks in that it is difficult to precisely adjust the position of the workpiece with respect to the laser beam, this being especially critical when the workpiece is rotated at a high speed of rotation.
Further, the corresponding welding process takes a long time if there are a large number of workpieces to be welded successively. Indeed, in the process, the workpiece has to be positioned in its still support before the latter is driven in rotation with a high speed. Then the welding treatment is applied as soon as the workpiece is rotated faster than a predetermined rotating speed. After the welding treatment, the support has to stop rotating before the workpiece is removed from it and before the next workpiece is positioned.
Other systems are known, in which the workpiece remains still during the welding process, the laser beam direction being changed to sweep the whole welding region. In order to cover the whole welding region with the laser beam, two mirrors are arranged in the optical path of the laser beam so that they can be tilted independently of each other, each between two end positions. Thus, one of the mirrors is responsible for a movement of the laser beam along a first direction X-X′ while the second mirror is responsible for a movement of the laser beam along a second direction Y-Y′, perpendicular to the first direction X-X′. Consequently, the combination of the respective tilts of the two mirrors covers the whole welding region of the workpiece.
However, a drawback of such systems is that the larger the dimensions of the welding region, the further the end positions are from one another for each of the mirrors. Thus, the time necessary for a mirror to tilt from one end position to the other is longer, which leads to a decrease in the overall speed of the welding process.
Futher, JP 60-236482A in the name of Mitsubishi Electric Corp. and published on Nov. 25, 1985, discloses an apparatus for generating a rotating laser beam comprising a large focal length lens standing in a still position as regards the apparatus and a first reflecting surface mounted on a rotating support for being driven in rotation. The light reflected by the first reflecting surface is directed toward a second reflecting surface and then toward a third reflecting surface.
However, all three reflecting surfaces described in this document are of planar shape in cross-section and introduce thus an error in the shape of the image focus point formed on the workpiece to be welded, as they are combined with a large focal length lens.